1. Technical Field
The embodiments disclosed herein relate to a power supply, and more particularly, to a flyback type switching power converter having a primary-side controller capable of responding to a detected drop in secondary voltage caused by a dynamic load change.
2. Description of the Related Arts
Pulse width modulation (PWM) and pulse frequency modulation (PFM) are conventional technologies used for controlling switching power converters to achieve output power regulation. This includes regulation for constant voltage and constant current output regulation. Conventional flyback power converters include a power stage for delivering electrical power from a power source to a load, a transformer in the power stage coupled between the power source and the load, a switch in the power stage that is coupled in series with the primary winding of the transformer, and a switch controller coupled to the switch for controlling the on-time and off-time of the switch using a control signal at an operating frequency of the switching power converter. The on-time and off-time of the switch can be modified by this controller based upon a feedback signal representing the output power, output voltage or output current. The energy from the power source is stored in the gap of the transformer when the switch is on and is transferred to the load when the switch is off. Regulation can be accomplished by, among other things, measuring the output current (or voltage) and providing the measured output current or voltage back to the primary side controller, which modifies the on-time and off-time of the switch accordingly.
In order to improve cost performance and reduce overall size, many commercially available isolated power supplies employ primary-only feedback and control. By sensing primary side signals during each “ON” and “OFF” cycle, the secondary output and load condition can be detected and thus be adequately controlled and regulated. This includes both constant voltage and constant current modes of operation. Furthermore, many electronic devices require the power supply to provide a controlled and regulated power source over wide operating conditions, adding to the difficulty of primary-side sensing and control. Portable devices such as smartphones and tablet computers are examples of such devices.
FIG. 1 illustrates a typical operating curve of the power supplies used to provide a controlled and regulated power source to these types of devices. There are three major operating conditions that are presented to the power supply. Two operating conditions occur while the electronic device is connected to the power supply. In the first operating condition, Constant Voltage Mode (CVM) 101, the power supply is required to supply a regulated DC output of a fixed voltage within a certain tolerance as shown by CVM range 104. CVM 101 generally indicates that the internal battery of the electronic device is fully charged and the fixed voltage output of the power supply provides the operating power for the electronic device to be operated normally.
In Constant Current Mode (CCM) 102, the power supply is required to provide a fixed current output. CCM 102 generally indicates that the internal battery of the electronic device is not fully charged and the constant current output of the power supply allows for the efficient charging of the internal battery of the electronic device. While operating in the CCM, the power supply is required to supply a regulated DC output of a fixed current within a certain tolerance as shown by CCM range 105. The third operating condition, No-Load 103, is when the electronic device is disconnected from the power supply. In No-load 103, the power supply is required to maintain a regulated voltage output in anticipation of the electronic device being re-connected to the power supply.
Because of convenience, it is common for end users to leave the power supply connected to the AC mains at times. Because it is necessary to maintain a regulated output voltage even in no-load conditions, a dual-mode control methodology is commonly employed. During the period when there is a nominal load, pulse width modulation is employed. When the load approaches no load, it is difficult to maintain a PWM duty-cycle low enough to maintain output regulation. A pre-load, or dummy load can be added, however, operational efficiency and no-load power consumption would be negatively impacted. Furthermore, because the power supplies are connected to the AC-mains even during long periods of time when they are not connected to the electronic device, government and environmental agencies have placed maximum limits on the no-load power consumption.
In these conditions, a common technique is for the controller to switch from PWM to PFM. Under no-load conditions, the rate of pulses driving the switch in the power stage is decreased significantly in order to maintain output voltage regulation, resulting in long periods of time between “ON” and “OFF” cycles. This presents a significant challenge to primary-side sensing control schemes that rely on the “ON” and “OFF” cycle to obtain a feedback signal. During the periods between “ON” time and “OFF” time, the status of the output voltage is unknown by the controller as there is no feedback signal available. Especially concerning is the event that the electronic device is reconnected to the power supply, representing a dynamic load change, during these long periods where the primary-side control is unaware of the state of the secondary output voltage. The dynamic load response in this case would be poor, causing the output voltage to drop accordingly. This may cause the undesired affect of the output voltage exceeding the regulation specifications.
FIG. 2 illustrates a conventional flyback power supply 200 with controller 201 employing primary-only feedback. Controller 201 has a feedback pin FB to obtain the secondary voltage 203 information reflected on the auxiliary winding 204, and controller 201 senses the reflected waveform from auxiliary winding 204 to obtain the voltage level of output 203. In order to maintain regulation of output 203 under light and no-load conditions, controller 201 may employ PFM and reduce the operating frequency of the drive signal 206 that controls the turn-on and turn-off of switch SW. Since controller 201 samples the reflected waveform in order to determine the output voltage level of output 203 during each “ON” and “OFF” cycle to sample, the reduction in operating frequency results in long periods of time when the output voltage is not monitored by controller 201. A sudden increase in load during these unmonitored periods causes a drop in the voltage of output 203 exceeding the regulation specifications.
FIG. 3 illustrates the associated waveforms of the power converter of FIG. 2. At time T_0 the operating frequency of the gate drive control signal 206 defined by controller 201 is reduced to a minimum Freq_Op_(MIN) in response to the output load at no-load (0 Amps) while the output voltage (V_OUT) is maintained at the regulated output V_REG. This operating mode is commonly referred to as “skip-mode”. At time T_1, the output load is dynamically increased to 100% rated load, causing the output voltage (V_OUT) to decline. However, since there is a long period before the next “ON” and “OFF” cycle, there is a long delay before controller 201 detects the drop in the output voltage. At time T_2, controller 201 initiates an “ON” and “OFF” cycle, at which time controller 201 detects the drop in the output voltage (V_OUT) and responds by increasing the operating frequency and/or “ON” time of the switch SW to respond to the increase in load. However, the long delay in detecting the increase in load may cause the output voltage to drop below the regulation limits V_REGLATION_MIN as shown in FIG. 3.